An increasing number of wireless networks are implemented as multiple-input, multiple-output (MIMO) systems that use multiple antennas to communicate with subscriber stations (also called mobile stations, mobile terminals, and the like). For example, MIMO antenna systems are used in code division multiple access (CDMA) networks, time division multiplexing (TDM) networks, time division multiple access (TDMA) networks, orthogonal frequency division multiplexing (OFDM) networks, orthogonal frequency division multiple access (OFDMA) networks, and others. In order to maximize throughput, MIMO networks use a variety of channel estimation techniques to measure the transmission channels between base stations of the wireless networks and mobile devices.
For example, a MIMO base station may transmit a first pilot signal from a first antenna and a second pilot signal from a second antenna. A subscriber station receives both pilot signals and uses each pilot signal to perform channel estimation for each antenna. The symbols of the pilot signals are transmitted on orthogonal subcarriers to prevent the pilot signals from interfering with each other. No data symbols are transmitted on the subcarriers of the pilot signals. If more than two antennas are used, each antenna transmits a separate pilot signal on a dedicated subcarrier that is orthogonal to the other pilot signal subcarriers. The drawback to this method is that a large amount of signaling overhead is used for channel estimation. This wastes bandwidth and reduces system capacity.
Besides achieving pilot orthogonality in the dimension of the subcarriers, the pilot orthogonality may also achieved in the dimension of time. A MIMO base station may transmit separate pilot signals from different antennas using time multiplexing to separate the pilot signals. For example, during a first transmit time, the base station may transmit a first pilot signal from a first antenna. Then, during a second transmit time, the base station may transmit a second pilot signal from a second antenna. If more than two antennas are used, each antenna transmits a separate pilot signal during a separate transmit time. The drawback to this method is that more time slots are needed for pilot signals, thereby reducing the number of time slots available for transmitting user data. As before, this reduces system capacity.
In some CDMA systems, multiple pilot signals are transmitted simultaneously from multiple antennas, but they are separated in the dimension of “code”. The pilots from all antennas are transmitted with preambles that use different pseudo-random noise (PN) codes. This reduces the number of time slots required for pilot signals. However, the drawback to this method is that the receiver circuitry in the mobile device must use complicated interference cancellation techniques to recover the MIMO pilot signals. This method reduces pilot signaling overhead at the cost of a more complicated pilot recovery scheme. This method also decreases the reliability of the channel estimates.
Therefore, there is a need in the art for an improved apparatus and method for performing channel estimation in a multiple-input, multiple-output (MIMO) wireless network.